ci: custom notification that Slack-mentions authors on failure

Created by: bobheadxi

This will replace our current default #buildkite-main messages with something very similar, but now includes a custom message that Slack-mentions authors (if the data is available) of builds that fail.

This also publishes our team library since I can't import from internal/team from within enterprise/dev/ci/internal/ci (not sure why tbh - don't really entirely understand Go's internal package rules, but this is probably fine)

Example notification: https://sourcegraph.slack.com/archives/C02FLQDD3TQ/p1643330812889309

Closes #25454 (closed) , requires https://github.com/sourcegraph/infrastructure/pull/3070